This invention relates to a system for measuring the dimensions of a cornea, either indirectly from a keratograph or directly from the cornea itself. In particular, the invention involves a system which uses computer-controlled digitization and analysis of video image information to provide fast, accurate corneal measurements.
For several decades, measurement of the human cornea has been obtained through the use of the keratometer. The keratometer measures an area 2.5 mm to 3.5 mm in diameter centered on the visual axis, and provides accurate measurements of the cornea's sagittal height and diameter, but is incapable of providing accurate readings of the intervening dimensions which are helpful to accurate diagnosis and contact lens prescription.
The Corneascope, the subject of U.S. Pat. No. 3,797,921 was developed to provide greater flexibility and accuracy in measuring the cornea. The Corneascope is designed to measure the diameter of the cornea using Placido rings which are spaced at increasing depths or increments starting at approximately 0.1 mm from the corneal apex. The Placido rings are reflected from the cornia and superimposed on a color photograph of the cornea to produce a keratograph of, typically, nine rings which are spaced from 0.1 mm to 3.5 mm from the corneal apex. See corneal photograph/keratograph 10 in FIG. 1 and the rings 11--11 therein. In terms of functional results, the Corneascope is an improved keratometer which provides a photograph of mire reflections at spaced depths along the corneal surface.
The Comparator described in U.S. Pat. No. 3,804,528 is used for analyzing the information provided by keratographs. The Comparator compares the keratograph of a patient's cornea to photographs of Placido rings reflected off base curves of known radii for the purposes of reading the corneal radii associated with the keratograph rings. The Comparator provides the optical zone diameter and base curves needed to fit a selected area of the cornea; clearance and touch points of the selected base curve; as well as the associated fluorscein pattern.
Among other benefits, the Corneascope and the Comparator provide a permanent photographic mold of the cornea and accurate determination of crucial corneal dimensions necessary for lens prescription, surgery and other diagnostic needs. Experience in using the Corneascope and the Comparator has indicated the need for a system which is an alternative to or an adjunct to the Comparator for even more fully and quickly quantifying the type of information contained in keratographs.
A computerized operating keratometer has been illustrated in Ophthalmology Times, vol. 8, No. 7, Apr. 1, 1983, p. 71, and is believed representative of computer systems which are used to quickly digitize and analyze an image of the cornea or a keratograph to thereby provide numeric measurements. The particular system comprises a surgical microscope which interfaces optically with a video camera and a ring light of an image processor, and a TV monitor.
One of the difficulties in using TV cameras or the like to analyze keratographs or corneas is the need to digitize an NTSC-standard TV image which comprises an array, shown in FIG. 2, of approximately 480.times.750 image points or pixels (each pixel being digitized to 6-8 bits of brightness resolution), approximately 360 kilobytes of image information. Furthermore, the TV imaging is performed at a scan rate of 30 frames per second and 525 lines per frame, for an overall scan rate of 15,750 lines per second. To suppress flicker, odd lines are scanned first (top to bottom) as one field and the even lines are scanned as a second field and interlaced with the odd lines. Effectively, then, there are two fields (one even/one odd) per frame. Automated processing of the TV image of a keratograph or cornea could be expected to require the digitization of the approximately 360,000 bytes of image information. In addition, such processing must contend with the high scan rate of the horizontal, vertical and field signals which are associated with the video image.
Accordingly, it is an object of the present invention to provide a system which is adapted to the above TV imaging constraints and which fully, quickly and automatically analyzes information of the type provided by keratographs.
It is another object of the invention to perform such enhanced analysis using keratographs.
It is still another object of the present invention to provide a system which provides such enhanced analysis of keratograph-type information and does so on a real time basis, directly from the cornea.
In one aspect, the system of the present invention comprises a video camera; means for positioning a keratograph before the camera; a monitor for displaying an image of the keratograph; a computer for converting digitized video information to numeric measurements and for correcting digitized video information; and an interface circuit for digitizing selected regions of the keratograph and transferring the associated data to the computer one scan line at a time.
In another aspect, the system includes a computer monitor adapted for displaying the numeric measurements.
In still another aspect, the above system includes a microscope which interfaces optically with the video camera for projecting a ring image on a cornea to thereby provide real time analysis of the cornea.
In still another aspect, the present invention relates to a method for determining the topography of a corneal surface, by providing a video corneal image comprising a matrix of scan lines and superimposed reference curves such as Placido rings or mires; digitizing the image matrix one scan line at a time for determining the intersectional position of the reference curves along at least one selected axis; verifying the accuracy of the detected reference line positions by displaying the reference line positions on the video image; and selectively positioning display cursors at the correct reference line position on the image for controlling selected correctional movement of the detected reference line positions.